Sounds like you have two choices.
Take away her decision-making ability (some kind of court declaration of incompetence) and manage her finances for her.
Or - resign yourself to the risk that she MIGHT screw up her finances, and will either be living at your house eventually, or you will have to pay for a home for her.
Unfortunately, if it's HER money and she is competent, it's not really any of your business if she makes smart or dumb choices. Nothing is more frustrating than watching people you love make dumb choices, but you cannot FORCE people to be reasonable if they don't want to be.
I'm not really in the same situation, since my parents are somewhat well off (and have a really good retirement). In fact, I actually continually encourage them to spend more money, because I don't want to fight with my asshole brother over any inheritance.